The present invention relates to an apparatus for driving an electro-luminescent device.
Electro-luminescent sheets are used in a variety of applications, including signs and house numbers. Electro-luminescent sheets typically comprise a conductive substance sandwiched between two conductive plates. The conductive substance becomes luminescent in the presence of an electric field, so that by applying a voltage across the two conductive plates, the conductive substance emits light.
Electro-luminescent sheets currently available require an AC voltage to be applied to the conductive plates in order to achieve luminescence. Inefficiencies in existing DC-AC inverter circuits have limited the use of electro-luminescent sheets where only battery power is available.
Typically, an electro-luminescent sheet requires 40-80 volts, depending upon the brightness required. Existing DC-AC inverter designs consume approximately 80 milliamps or more to achieve the required AC voltage.
Throughout this specification, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d or variations thereof, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
According to one aspect of this invention, there is provided an apparatus for driving an electro-luminescent device, characterized by: a transformer including a ferrite core and a first primary winding, a second primary winding and a secondary winding wound onto said core without an air gap; one end of each of the first and second primary windings being connected together; an electrical circuit arranged to drive said one ends of the first and second primary windings; a feed back circuit connected between the electrical circuit and the other end of the second primary winding to control operation of said electrical circuit; one end of the secondary winding being connected to said other end of the second primary winding; and the other end of the secondary winding being connected to the electro-luminescent device.
Preferably, the electrical circuit includes a transistor,
Preferably, the feed Pack circuit comprises a resistor and a capacitor provided in series between said other end of the secondary primary winding and the base of the transistor.
Preferably, the resistor and the capacitor are arranged such that the transistor produces an oscillating output having a frequency less than 200 Hz.
Preferably, the apparatus includes a timing circuit arranged to deactivate said electrical circuit after a predetermined time period.
Preferably, the apparatus further includes a light sensing circuit arranged such that said electrical circuit is deactivated in the presence of daylight or other strong light sources.
Preferably, the turns ratio between the first and second primary windings is between 1:14 and 1:12. More preferably, the turns ratio between the first and second primary windings is 1:13⅓.
Preferably, the second primary winding has 16.5 turns.
Preferably, the turns ratio between the second primary winding and secondary winding is between 1:100 and 1:250. More preferably; the turns ratio between the second primary winding and the secondary winding is between 1:175 and 1:185. Still more preferably, the turns ratio between the second primary winding and the secondary winding is between 1:181 and 1:182.
Preferably, the secondary primary winding is wound onto said core first, then said second primary winding and finally said first primary winding.
According to a second aspect of the present invention, there is provided an electro-luminescent device, characterized in that said device comprises at least in part a driver apparatus, said driver apparatus in turn comprising: a transformer including a ferrite core and a first primary winding, a second primary winding and a secondary winding wound onto said core without an air gap; one end of each of the first and second primary windings being connected together; an electrical circuit arranged to drive said one ends of the first and second primary windings; a feed back circuit connected between the electrical circuit and the other end of the second primary winding to control operation of said electrical circuit, one end of the secondary winding being connected to said other end of the second primary winding; and the other end of the secondary winding being connected to the electro-luminescent device.
According to a further aspect of this invention, there is provided an apparatus for driving an electro-luminescent device, characterised by: a transformer, an electrical circuit arranged to drive the transformer; a feed back circuit connected between the electrical circuit and the transformer to control operation of said electrical circuit; a light sensing circuit; and a timing circuit arranged to deactivate said electrical circuit after a predetermined time period.
Preferably, the light sensing circuit comprises at least in part a light dependent resistor and a resistor arranged to provide bias voltage to a NAND gate. The light dependent resistor and the resistor bias both inputs of the NAND gate where NAND gate acts as an inverter. Still preferably, the light dependent resistor and the resistor are arranged such that in the presence of strong light the output of the NAND gate will be low and in darkness the output of the NAND gate will be high.
The output of the NAND gate charges or discharges a capacitor via a resistor. A diode is connected between the capacitor and the positive voltage rail to clamp the voltage across the capacitor. The capacitor, the resistor and the diode form a damping circuit for the output of the NAND gate.
A NAND gate is connected to the capacitor. The output of the NAND gate acts as an input to a NAND gate. A capacitor and a resistor are connected to the output of the NAND gate to form a high-pass filter. The voltage across the resistor is output to the timing circuit.
The capacitor and the resistor are chosen such that the output of the NAND gate must be high for 4 minutes before the voltage across the capacitor is sufficient to trigger the NAND gate. This is desirable to prevent temporarily bright lights, such as those from passing vehicles, re-triggering the timing circuit. The capacitor and the resistor act to provide a pulse signal to the timing circuit.
The timing circuit comprises an integrated timing circuit, a crystal, a diode, a resistor and a capacitor. The integrated timing circuit is connected to the resistor and receives the pulse signal therefrom. The crystal is connected to the integrated timing circuit to provide a stable frequency source. The integrated timing circuit is arranged to provide a pulse output for 7 hours after being triggered. The diode, the resistor and the capacitor act as a smoothing circuit such that whilst the integrated timing circuit is providing a pulse output, the voltage across the capacitor is high.
The control circuit comprises a transistor biased by resistors and connected between the position voltage rail and the output of a NAND gate such that the transistor will be switched on only when the output of the NAND gate is low. The collector of the resistor is connected to the base of the transistor via a resistor.
One of the inputs of the NAND gate is connected to the output of the NAND gate and the other input of the NAND gate is connected to the capacitor. Since both inputs must be high in order that the output of the NAND gate be low, and consequently the transistor be switched on, power will not be supplied to the electro-luminescent sheet unless the timing circuit is active and light is not sensed by the light dependant resistor